US-South Korea Collaborative Research: Additive Manufacturing of Fatigue Resistant Materials

添加制造或3D打印提供了制造定制的复杂金属部件的能力，这些部件传统上无法用于各种应用。因此，工程设计和产品实现的范式正在发生转变，许多行业，如生物医学和航空航天，都将受益。一些例子包括(1)现场快速制造具有患者和损伤特定设计的金属骨植入物，以及(2)在偏远地区(如外层空间)制造替换部件。尽管如此，目前的添加剂制造方法制造的金属零件往往具有孔洞和各向异性，这些特征通常不利于零件的强度和抗疲劳性能。此类部件不能在承重应用中放心使用。该奖项支持可能通过添加制造来生产耐疲劳金属部件的科学研究。本研究的目的是(1)建立微观组织性能(晶向和形貌)、气孔率分布(尺寸、形状和位置)和工艺参数(激光功率、扫描速度、填充间距和层取向)之间的关系；以及(2)了解添加制造的材料的组织性能(晶向)和孔隙率对其多轴疲劳抗力的影响。为了实现第一个目标，将开发连续介质尺度的热物理模型，并将其用于将微结构特性和孔隙率分布与工艺参数联系起来。这些模型将得到实验验证。Ti-6Al-4V试件将在不同的工艺参数组合下，采用激光加法制造。层的方向将在0？-90？之间变化，而扫描速度、阴影间距和激光功率将在现有知识(例如，已发表的实验数据)规定的范围内变化。将使用X射线层析成像(孔洞的大小、形状和位置)以及光学和扫描电子显微镜(颗粒取向和形态)来测量所制备的试件的微观结构特性和孔隙率分布。为了实现第二个目标，将建立基于临界平面方法的多轴组织敏感疲劳模型，并进行实验验证。多轴疲劳试验将在制作的试件上进行，使用同相和异相区分加载路径来行使不同的临界加载面。将对试件的断口进行断口分析，以确定引起裂纹的气孔(S)的位置、大小和形状。裂纹复制技术将被用来寻找疲劳微裂纹相对于临界载荷面的方向，并确定各向异性组织对疲劳行为的影响。通过与韩国工业技术研究所的合作，生成的模型将在其他添加剂制造方法和材料上进行测试。

Additive manufacturing, or 3D printing, offers the ability to fabricate customized, complex metallic parts traditionally unobtainable for a variety of applications. A paradigm shift in engineering design and product realization is thus occurring, and many industries, such as biomedical and aerospace, are poised to benefit. Some examples include (1) on-site, rapid fabrication of metallic bone implants with patient and injury-specific designs, and (2) fabrication of replacement parts in remote locations (e.g. outer space). Nonetheless, metallic parts made by current additive manufacturing methods tend to have porosity and anisotropy, features typically detrimental to part strength and fatigue resistance. Such parts cannot be used with confidence in load-bearing applications. This award supports scientific investigation that can potentially enable production of fatigue resistant metallic parts by additive manufacturing. The objectives of this research are (1) to establish relationships between microstructure properties (grain orientation, and morphology), porosity distribution (size, shape, and location), and process parameters (laser power, scanning speed, hatch spacing, and layer orientation); and (2) to understand effects of microstructure properties (grain orientation) and porosity of additively-manufactured materials on their multi-axial fatigue resistance. To achieve the first objective, continuum-scale thermophysical models will be developed and used to relate microstructure properties and porosity distribution to process parameters. These models will be experimentally validated. Ti-6Al-4V specimens will be fabricated using laser-based additive manufacturing under various process parameter combinations. The layer orientation will be altered between 0º-90º, while scanning speed, hatch spacing, and laser power will be varied within ranges prescribed from existing knowledge (e.g. published experimental data). Microstructure properties and porosity distribution of fabricated specimens will be measured using X-ray tomography (size, shape, and location of porosity), as well as optical and scanning electron microscopy (grain orientation, and morphology). To achieve the second objective, multi-axial microstructure-sensitive fatigue models based on critical plane approaches will be developed and validated by experiments. Multi-axial fatigue experiments will be conducted on fabricated specimens, using in-phase and out-of-phase discriminating load paths to exercise different critical loading planes. Fractography on the fracture surface of specimens will be performed to determine location, size, and shape of the pore(s) responsible for initiating cracks. Crack replication techniques will be employed to find the orientation of fatigue micro-cracks with respect to critical loading plane and to determine effects of anisotropic microstructure on fatigue behavior. Through the collaboration with the Korea Institute of Industrial Technology, the generated models will be tested on other additive manufacturing methods and materials.